The MFLL is an airborne lidar system, a single-beam multi-pixel laser altimeter using PRN (Pseudo Random Noise) modulated fiber lasers. The instrument has been developed under a NASA ESTO (Earth Science Technology Office) IIP (Instrument Incubator Program) grant for Climate Change research and Exploration. The major goal of MFLL is to demonstrate the required lidar technology concept for the LAS (Laser Absorption Spectrometer) to be flown on the future spaceborne ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) mission of NASA. The airborne MFLL is the prototype of the future LAS instrument, it is also referred to as ITT's CO2 EDU (Engineering Development Unit) for ASCENDS. ITT Geospatial Systems of Fort Wayne, Indiana is the developer of the instrument. NASA/LaRC (Langley Research Center) in Hampton, VA is the operator of the MFLL instrument and is providing flight data analysis. 1)2)3)4)5)

Note: ASCENDS is a NASA mission, recommended for launch in the second phase of missions by the 2007 NRC (National Research Council) Earth Science Decadal Survey. It is considered the technological next step following launch of the NASA OCO-2 (Orbiting Carbon Observatory-2) mission planned for launch in 2013. Using an active laser measurement technique, ASCENDS will extend the CO2 remote sensing capability to include uninterrupted/all-season coverage of high-latitude regions and nighttime observations with sensitivity in the lower atmosphere, to enable investigations of the climate-sensitive southern ocean and permafrost regions, provide insight into the diurnal cycle and plant respiration processes, and provide useful new constraints to global carbon cycle models.

MFLL instrument:

The instrument was built largely with off-the-shelf components and uses high reliability telecommunication components, including lasers, modulators and fiber amplifiers in the transmitter. All wavelengths are transmitted simultaneously from a single fiber collimator and the return signal is collected by a simple telescope (20 cm aperture) fiber coupled to a HgCdTe APD (Avalanche Photodiode).

This eliminates the sensitivity to common electronics noise and highly varying surface reflectance, and also minimizes the effects of atmospheric turbulence and speckle by making them common mode. The analog signal is sampled with a high resolution scope card housed in a National Instruments PXI chassis and the digitized signal is passed through the custom-built software-based lock-in processing system which allows separation of the signals from the individual wavelengths. The separated signals are then used in the standard DIAL (DIfferential Absorption Lidar) relations to determine the integrated column differential optical depth.

The MFLL system is composed of 3 major parts: the transmitter, receiver, and C&DH (Control and Data Handling) subsystems shown in Figure 1.

A block diagram of the MFLL is shown in Figure 2. The MFLL transmitter consists of four DFB (Distributed Feedback) lasers (only two shown in Figure 2) which are set to the desired wavelengths in the 1.57µm region, and these wavelengths are continuously monitored with a heterodyne process that is referenced to a gas cell. The amplitude of each laser beam is then electro-optically (EO) modulated at a unique frequency for the three LAS CO2 wavelengths (one on the CO2 absorption line and two off the line on opposite sides) or has an imparted code for the one PRN altimeter channel. An Erbium Doped Fiber Amplifier (EDFA) amplifies the combined LAS laser beams at the same time to a combined average power of 5 W, and a separate EDFA is used for the PRN altimeter to also produce a 5 W output beam.

A reference tap is used to monitor the outgoing power in each laser channel (R). Since all of the LAS beams are transmitted simultaneously out of the single fiber collimator, they have 100% spatial and temporal overlap. This eliminates sensitivity to highly varying surface reflectance as well as minimizing effects of atmospheric turbulence by making it common mode.

The backscattered returns from the surface or cloud top for both the LAS and PRN altimeter are collected by a single telescope. The LAS and PRN altimeter returns are dichroically separated, and all the LAS returns are sent to the same low noise HgCdTe avalanche photodiode where the optical signal is converted to an analog voltage signal and then is processed through a lock-in amplifier to separate the individual signals (S) for the LAS channels. The energy normalized LAS return signals (S/R) are ratioed to yield the differential transmission of the transmitted wavelengths. The natural log of the differential transmission is directly proportional to the CO2 column amount.

The PRN altimeter return is routed to a photomultiplier tube where the signal is amplified and analyzed to determine the range to the surface/cloud (L) and the relative intervening aerosol scattering profile and surface reflectance. The differential CO2 absorption cross section (Δσ) is calculated from information on the laser wavelengths and atmospheric pressure and temperature along the measurement path.

The MFLL is currently configured to transmit up to three laser lines; however, the architecture is expandable to transmit more lines depending on the desired spectroscopic configuration. Several laser lines have been used for CO2 column measurements: one at the center of the R24 CO2 absorption line, one offset by +10 pm from line center, and one offset by ±50 pm on either side of the line. The off-line is considered to be at the 50 pm offset position, and the on-line is either at line-center or the side-line position at +10-pm offset. The side-line operation gives additional weighting of the CO2 column measurement to mid-to-lower troposphere when the measurement is from high altitude or from space. From a low to medium altitude platform, there is little advantage of using the side-line measurement, and as a result the MFLL flight tests use the line-center for the on-line measurements.

The CO2 LAS detects the scattered laser light from the Earth's surface or cloud tops, and the differential optical depths are calculated from the ratios of the on-line return signals at line-center or side-line to the off-line return signals at ±50 pm offset. Assuming the measurement of the outgoing laser energy has a signal-to-noise ratio (SNR) at least ten times larger than the surface/cloud return signals, the CO2 column measurement precision (dn/n) is related to the SNR of the on-to-off return signal ratio and the one-way differential optical depth (τ) to the surface by the relationship: dn/n~(-1/(2τ x SNR)). Thus, to make a CO2 column measurement with the equivalent of 1 ppmv uncertainty from space would require the on-to-off ratio to have a SNR of ~110 at line-center with τ ~1.8 or a SNR ~940 at the side-line with τ ~0.41. These SNR ratios would need to be achieved with an integration time of 10 s (horizontal distance of ~70 km).

MFLL flight test campaigns:

ITT’s CO2 LAS has been operationally validated via extensive ground and aircraft campaigns in cooperation with NASA/LaRC. Eight flight test campaigns of the MFLL instrument have been conducted since May 2005.

The flight validation campaigns were designed to permit validation tests under a number of measurement conditions. LAS flight legs of 50-100 km in length were flown multiple times at several different altitudes to determine the consistency of the remote CO2 measurements and their correlation with altitude. In addition, in situ CO2 profiles were obtained from as low an altitude as possible to the LAS operational altitude on spirals at the beginning and end of the flights with the entire flight lasting about 2.5 hours. The flights were conducted both during the day and at night, over a wide range of surface conditions (land and water), and in clear and scattered-cloud conditions.

In addition to the campaigns listed above, a major ASCENDS flight test campaign was conducted using the NASA DC-8 during July 6-18, 2010. The MFLL system and associated in situ CO2 instrumentation were operated on DC-8 flights over the Central Valley of California, the desert of southeastern California/Nevada, the Pacific Ocean off of the Baja Peninsula, Railroad Valley, Nevada, and the DOE ARM CF in Lamont, Oklahoma. Remote CO2 column measurements were made from altitudes of 2.5 to 13 km, and in situ CO2 profiles were obtained on spirals from the highest altitude on each flight to as low as 30 m at the center of the flight track. Radiosondes were also launched in conjunction with these flights to constrain the meteorological conditions for the validation of the MFLL CO2 column measurements. 6)

The high-precision, high-accuracy remote CO2 measurements obtained by the MFLL system represent a major step towards the realization of the needed capability for future space-based laser measurements of the global distribution of CO2.

Results from eight flight tests conducted over various land and water regions of Virginia between October 17-23, 2007 are shown in Figure 4. The MFLL measurements of CO2 ODs (Optical Depths) were found to be highly correlated with platform altitude with an R2 = 0.995, and a comparison of the MFLL measured ODs with the ODs derived from the in situ CO2 measurements and the CO2 spectroscopy also showed a high correlation with an R2 = 0.996. These results were obtained under widely different atmospheric, surface, and background conditions. The average difference between measured and modeled ODs was found to be less than 0.33% or the XCO2 equivalent of about 1.25 ppmv.

During the flight tests in September-October 2008, improvements in the in situ sampling strategy were implemented, and the average difference between the measured and modeled CO2 ODs was found to be 0.11% or 0.42 ppmv with a standard deviation of 0.49% or 1.9 ppmv.

Legend to Figure 4: The left plot shows the linear correlation of MFLL CO2 optical depth (OD) measurements with respect to aircraft altitude, and the right plot shows the linear correlation between the remote and in situ (modeled) CO2 ODs.

A comprehensive multiple-aircraft flight test program was conducted over Oklahoma and Virginia in July-August 2009. An example of the MFLL-derived surface reflectance and average CO2 column variations along the 50 km flight leg from an altitude of 4.6 km over the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Central Facility (CF) in Lamont, Oklahoma on July 31, 2009 is shown in Figure 5.

A 1 s average was used for these data, and the average XCO2 column was calculated from the MFLL measured CO2 OD using radiosonde data taken at the CF and the CO2 spectroscopic parameters for the LAS measurements. The surface reflectance varies over short distances by a factor of 2 to 3 along the leg, but the variations in XCO2 are generally contained within ±1 ppmv of the average value of 378 ppmv. The average MFLL XCO2 SNR for 1 s horizontal averages along the leg was found to be 760, which is a XCO2 precision of 0.60 ppmv, and for a 10 s average the SNR was 2002 or 0.20 ppmv. On a subsequent flight over the Chesapeake Bay from 4.6 km on 12 August, the 10 s XCO2 SNR was found to be 1300 or 0.30 ppmv.